Janáček (Leoš) is more than just another excuse to make Blogger spit out weird characters. He was a very important Czech composer of the early 20th century. Many of his pieces are regularly performed, including two string quartets, the Glagolitic Mass, the tone poem Taras Bulba, the Sinfonietta, and several operas, including (but not limited to) The Cunning Little Vixen and The Makropoulos Affair.
Jolivet (André) was a 20th century French composer who combined experimental modernism with references to ancient music. He wrote a number of concertos for different instruments and combinations of instruments, such as "bassoon, strings, harp, and piano," "flute and percussion," "harp and chamber orchestra," "trumpet, strings, and piano," and even an electronic instrument called the ondes Martenot.
Joplin (Scott) was a celebrated African-American composer, mainly known for his ragtime piano pieces such as The Entertainer and The Maple Leaf Rag. He also wrote an opera called Treemonisha, a fusion of ragtime and classical music which was not performed until 45 years after Joplin's death. And...wow! For several years Joplin lived and worked right here in St. Louis!
Josquin des Prez was a Flemish composer of the 16th century. Martin Luther admired his music, and for good reason; he was one of the very best handful of Renaissance composers. What student of either choral music or early music hasn't heard and/or performed at least one of Josquin's songs, such as El grillo or Nymphes de Bois? He wrote masses, sacred motets, and mostly French secular songs in a sophisticated polyphonic style that, at the same time, magically painted the meaning of the text in musical colors.
Joubert (John) is a British composer who has written some symphonies, operas, and concertos, though he is best known for his choral music, including the Herefordshire Canticles, Torches, There Is No Rose of Such Virtue, and O Lorde, the Maker of Al Thing.